The process of adding calcium into liquid steel to modify oxide and sulfide inclusions and thus improve steel quality has been generally accepted by persons in metallurgical field. At present, the technique has been widely used in pipeline steel, gear steel, weathering-resistant steel, free-cutting steel stainless steel, electrical steel and other high-end products, so as to improve the corrosion resistance, microstructure, mechanical property, manufacturability, and electromagnetic performance, etc.
Calcium does not dissolve in liquid steel, and has a low melting point (850° C.) and a low boiling point (1,483° C.). And it is easy to form calcium steam which exists in the form of bubbles inside liquid steel. Calcium also has a strong deoxidizing and desulfurizing capacity, and may react with the oxygen and sulfur in liquid steel to form complex sulfides, calcium aluminates and other inclusions. On one hand, it is easy for these calcium oxide-enriched particles formed during deoxidation to separate from the melting pool; on the other hand, when the melting pool is stirred, the solid calcium oxide inclusions in liquid steel may be modified so as to reduce the melting point of the inclusions, facilitate their polymerization, growth and floating upward, and improve the purity of steel.
Generally, calcium treatment is conducted in the atmospheric status to avoid the excessive loss of calcium. Such calcium treatment methods include wire feeding method (CaFe, CaSi), blowing method (CaSi, CaO) and shooting method (CaFe, CaSi). At present, these techniques are relatively mature and easy to operate, which play an important role in industrial production. However, applying these techniques usually increase the smelting treatment cycle, lead to significant temperature drop in the treatment process and cause secondary pollution problems (like oxygen uptake, nitrogen uptake, entrapped slag, etc.) due to the boiling of liquid steel, which are unfavorable for the stable improvement of steel purity and production efficiency.
Among these techniques, the relatively representative calcium treatment methods include the following methods:
In the Japanese laid-open Patent Publication No. 1996-157932, in the atmospheric status, liquid steel is added with calcic materials after deoxidation by the input method. The patent points out that the addition amount of calcic materials depends on the content of silicon oxide in the slag. Appropriate calcium treatment can improve the steel quality defect of finished strip steel products caused by the large amount of inclusions.
In the Japanese laid-open Patent Publication No. 2009-57612, in the atmospheric status, liquid steel is added with CaSi wire by the wire feeding method, wherein the yield of calcium can reach as high as 6.7% at a wire feeding rate of 100 m/min. However, at the end of wire feeding, the violent boiling of liquid steel may cause relatively significant secondary pollution.
In order to prevent the increase of oxygen and nitrogen of liquid steel caused by the calcium treatment by the wire feeding method, the Japanese laid-open Patent Publication No. 1996-157935 makes technical improvement to the technique. Before the wire feeding operation, the pre-tapped steel ladle cover is placed on the steel ladle so as to avoid the thorough exposure of liquid steel to the atmosphere.
In order to further improve the production efficiency and reduce fluctuations in the steel making production process, some technicians have also tried to provide calcium treatment for liquid steel in the RH (Ruhrstahl-Heraeus) refining process. The calcium treatment mainly includes the following treatments.
In the Japanese laid-open Patent Publication No. 1999-92819, in the vacuum status, liquid steel is added with calcium metal, calcium alloy and calcium oxide-aluminum oxide alkaline solvent mixture by the blowing method to generate diversified calcic complex inclusions, and also reduce the nitrogen content of liquid steel after vacuum treatment. It shall be pointed out that the complex addition of the above materials is required to reach a relatively satisfactory effect of inclusion control. Further, the actual treatment effect of liquid steel depends on the degree of their mixing and reaction in liquid steel and the status of liquid steel. However, the method has its own disadvantage: liquid steel needs to be added with calcium metal, calcium alloy and calcium oxide-aluminum oxide alkaline solvent mixture, and such mixture is produced at a relatively high cost by complex production processes, etc.
In the Japanese laid-open Patent Publication No. 1998-245621, in the vacuum status, liquid steel is uniformly fed with calcic materials by virtue of the circulation of liquid steel by the wire feeding method, so as to ensure a relatively satisfactory effect of inclusion control. The disadvantage of the method lies in that, the wire feeding method employed for calcium treatment usually causes significant environmental pollution, influences the circulation of liquid steel in vacuum and thus makes it difficult to either ensure the actual treatment effect of liquid steel or get the circulation mode under control, which as a result influence the normal treatment cycle of RH refining, and imposing relatively high requirements on the conditions of wire feeding equipment.
In some papers, in the vacuum status of the laboratory, liquid steel is added with calcium and iron alloy to study the change of inclusions in liquid steel. They point out that, by such calcium treatment method, the total oxygen content of steel is reduced, however, the amount of inclusions is increased and their average size is reduced. Thus, it is applicable only for DI and other special steel types.
Therefore, at present it still needs a method for the calcium treatment of non-oriented electrical steel sheet with relatively low cost, simple production process, convenient and controllable equipment, getting the form and amount of inclusions under control, and without influencing the normal treatment cycle of RH refining.